PROJECT SUMMARY/ ABSTRACT Early detection of breast cancer with mammography screening reduces cancer mortality by 15%; however, mammography screening does not perform equally well among all women. In particular, breast cancers are less likely to be seen on screening mammography among women with dense breasts. To make women aware of the known limitations of mammography in women with dense breasts, state specific legislation has been introduced to encourage dialogue between women and their physicians about supplemental screening options such as whole-breast ultrasound (US), magnetic resonance imaging (MRI), and digital breast tomosynthesis (DBT). As of October 2015, 48% of states have adopted legislation that requires women undergoing mammography to be notified of their mammographic breast density (MBD). The long-term goal of this study is to understand how supplemental screening among women with dense breasts is being utilized given the widespread attention to breast density legislation across the country. The objective of this application is to determine if the North Carolina breast density law impacts screening behavior in a diverse population and to elucidate how breast imaging facilities across the state are interpreting and implementing the law. We hypothesize that the use of supplemental screening among women with dense breasts will increase after the passage of the North Carolina breast density law on January 1, 2014 and that the use of supplemental screening will differ by patient, radiologist, and facility characteristics. We will test our hypothesis and accomplish the objective of this application by using existing data from the Carolina Mammography Registry (CMR) and a statewide facility survey to pursue the following specific aims: (1a) evaluate whether the use of supplemental screening differs by breast density pre- and post- passage of the North Carolina breast density law; (1b) identify patient-, radiologist-, and facility- level factors associated with the use of supplemental screening in women with dense breasts; (2) evaluate how mammography facilities responded to the passage of the breast density notification law in terms of onsite supplemental screening capacity and education to women regarding MBD. No prior studies have assessed the impact of breast density notification laws on a population-based sample of women undergoing screening mammography or the extent to which other factors in conjunction with MBD influence the use of supplemental screening. The proposed study is innovative because it leverages the existing infrastructure of the CMR to effectively determine the impact of the breast density law. We anticipate making a significant contribution by providing insight into the use of supplemental imaging pre- and post- passage of the breast density legislation in North Carolina.